A number of radio systems use transmission of frequency modulation signals to communicate across a broad range of radio frequencies. A majority of the frequency modulation generation circuits, for example modulator circuits, exciter circuits, used in such radio systems are alternating current (AC)-coupled. A preferred arrangement for generating frequency modulation signals is to use a two port modulation technique, whereby synthesisers and VCOs in such circuits are fed with two signals. The first, a VCO modulation input signal (vco.sub.-- mod), modulates the higher frequencies and a second, a compensation input port signal modulates the lower frequencies close to DC (ref.sub.-- mod). A benefit of using such a two-port modulation technique is that the modulation frequencies can be generated much more accurately down to fractions of a Hertz.
For the frequency generation circuit to also transmit data transmissions, such as telegrams for the European Paging standard (POCSAG), where the modulation scheme is Direct Frequency Shift Keying (DFSK), such a DFSK modulation technique is not capable of producing the required frequency deviation of the carrier signal. A DFSK frequency generation circuit requires a direct current (DC) signal FM response for the synthesiser's reference frequency in addition to the VCO-port modulation, in generating a final frequency. For this reason a reference oscillator with a DC response warp input is required. To provide a well balanced modulation signal (ref.sub.-- mod and vco.sub.-- mod) which shows a real DC response FM, a sufficiently large warp range, for example +/-40 parts per million (ppm) is needed. However, typical reference oscillators have a limited warp range of say, +/-10 ppm. Hence, this warp, used for generating standard frequency modulation signals, is not sufficient to also create a DC response output deviation required for DFSK generation. Dividing down or multiplying up the reference oscillator frequency will not affect the ppm warp range. The ppm ratio is therefore fixed.
A further problem occurs in the case where longer data telegrams are transmitted with, for example, a "worst case" series of bit patterns--successively weighted either to "1" or weighted to "0", thereby providing a one-sided modulation deviation causing the carrier frequency to drift towards one of the adjacent channels. This is due to the fact that such circuits are alternating current (AC)-coupled.
Thus there is a requirement to provide a frequency generation circuit to facilitate such modulation schemes and/or data transmissions of this type. Furthermore, there is a requirement to provide a frequency generation circuit that is capable of operation with both current frequency modulation transmissions and additional/enhanced transmissions by for example scaling warp input ppm values, such as those described hereinbefore.